Alkaline electrochemical cells, such as alkaline zinc-manganese dioxide batteries, include zinc powder as a negative electrode active material, and use an alkaline electrolyte, such as aqueous potassium hydroxide. Good physical contact between the zinc powder grains is necessary for good current collection. Furthermore, corrosion of the zinc in the presence of impurities dissolved in the alkaline electrolyte can produce hydrogen gas.
Historically, mercury was used as an alloying additive to the zinc powder in alkaline cells. The formation of the zinc amalgam with mercury improves the contact between zinc powder grains, improving current collection and reducing vibration sensitivity of the alkaline cells. The zinc amalgam with mercury also increases the hydrogen overvoltage, inhibiting corrosion of the zinc and hydrogen gas formation.
In an effort to eliminate mercury from alkaline cells, other additives have been used in its place. Indium has commonly been used to replace mercury. Unlike mercury, however, indium is not a liquid at room temperature. To improve contact between the zinc powder grains, the indium is introduced into the electrolyte as an indium compound, for example, as an oxide or hydroxide. The indium compound dissolves into the electrolyte, and subsequently plates out of solution to coat the zinc powder grains, and cements the grains together. Other metals are also alloyed with the zinc, to further enhance its properties.
Bismuth has also been used to replace mercury in alkaline cells. Bismuth has been introduced into the electrolyte as a bismuth compound, for example, as an oxide or hydroxide. As with indium, the bismuth compound dissolves into the electrolyte, and subsequently plates out of solution to coat the zinc powder grains, and cements the grains together. See, for example, U.S. Pat. No. 5,721,068.
Corrosion of zinc powder in the presence of an alkaline electrolyte is catalyzed by metal impurities which contaminate the latter as battery storage time increases. Since bismuth can form alloys with many of these metals, the bismuth captures the metal impurities which may be present in the electrolyte, as it plates out onto the zinc powder. This further reduces the corrosion of the zinc powder and hydrogen gas production. See, for example, U.S. Pat. Nos. 5,541,021 and 6,461,768.
The effect of particle size of the bismuth compound when used in an alkaline manganese dioxide cell containing zinc has been studied. See, for example, U.S. Pat. No. 6,258,484. Bismuth oxide, hydroxide and metal particles having an average particle size of 2 to 4 μm and a specific surface area of 1.2 to 2.4 m2/g (BET method), were compared to particles having an average particle size of 12 to 24 μm and a BET specific surface area of 0.4 to 0.8 m2/g. It was found that the smaller particles having an average particle size of 2 to 4 μm and a BET specific surface area of 1.2 to 2.4 m2/g, all produced the same results, and were superior in performance as compared to the larger particles.